Acoustic touch position sensors are known to include a touch panel or plate having a group of transmitters positioned along a first edge of the panel for simultaneously generating Rayleigh waves that propagate through the panel in a X direction to a group of detectors positioned on a second edge of the panel opposite to the first edge. A group of transmitters is also positioned along a third edge of the panel for simultaneously generating Rayleigh waves that propagate through the panel in a Y direction to a group of detectors positioned on a fourth edge of the panel opposite to the third edge. Interruption of intersecting waves by touching the panel causes unique output signals to be developed at a X detector and a Y detector defining the point of intersection. Such an acoustic touch position sensor is shown in U.S. Pat. No. 3,673,327.
Acoustic touch position sensors are also known wherein only two transducers per axis are required. For each axis one transducer imparts a surface acoustic wave that propagates along the perpendicular axis on which a first reflective grating is disposed to reflect portions of the surface acoustic wave along plural parallel paths of differing lengths to a second reflective grating. The second reflective grating reflects the surface acoustic waves to a second transducer where the signals are received for processing. The reflective gratings associated with the X axis are perpendicular to the reflective gratings associated with the Y axes so as to provide a grid pattern to enable coordinates of a touch on the plate to be determined. Acoustic touch position sensors of this type are shown in U.S. Pat. Nos. 4,642,423, 4,644,100, 4,645,870, 4,700,176, 4,746,914 and 4,791,416.
Acoustic touch position sensors utilizing surface acoustic waves as taught by the above-mentioned patents have a number of problems which are more readily understood when the nature of the surface acoustic wave used in these sensors is considered. If as in the above mentioned patents, the touch plate consists of a uniform, non-piezo electric medium, and the acoustic wave is confined at or near a single surface such as an outer surface of the touch plate, the surface acoustic wave is known as a Rayleigh wave. These waves have X and Z components such that disturbed particles move elliptically in the X-Z plane. It is characteristic of these waves that the disturbance decays rapidly with depth, that is in the -Z direction, so that the wave energy is essentially confined at or near the surface of the touch plate. Strictly, Rayleigh waves exist only in an infinitely thick medium. Waves in a uniform, non-piezoelectric medium of finite thickness that are confined to a single surface as shown in FIGS. 1A-1D are more precisely termed quasi-Rayleigh waves. Given a long enough propagating path in a medium of finite thickness, Rayleigh wave energy will not be confined at or near a single surface, but will transfer back and forth between the outer surfaces of the plate. A touch sensor according to the above mentioned patents, would be inoperable under these conditions because a touch in a region of one outer surface where complete transference of the wave to the opposite outer surface has taken place, will not disturb the wave, and is therefore undetectable. In practice, in order to provide a wave that is confined to a single surface, the thickness of the touch plate must be at least three to four times the wavelength of the wave imparted into the substrate, wherein the length and breadth of the touch plate are also limited.
If the thickness of the touch plate is for example two Rayleigh wavelengths or less, the waves emanating from the source transducers utilized in the above patents are clearly distinguishable from Rayleigh/quasi Rayleigh, and other surface acoustic wave (SAW) modes and are known as Lamb or Plate waves as shown in FIGS. 1E and 1F. Lamb waves exist in two groups of various orders, all of which propagate independently of one another. One group is characterized by particle displacement that is symmetric with respect to the median plane of the plate. The other group of Lamb waves is characterized by particle displacement that is anti-symmetric with respect to the median plane. In general a specific order within the symmetric Lamb wave group differs in phase and group velocity from the identical order of the anti-symmetric Lamb wave group. In particular with a plate thickness of approximately equal to two Rayleigh wavelengths, two modes of approximately equal amplitude are mainly excited, the zeroth order symmetrical Lamb waves and the zeroth order anti-symmetrical Lamb waves. As seen in FIGS. 1E and iF, the symmetrical and anti-symmetrical Lamb waves are not confined to a single surface of the touch plate, but extend through the plate to the opposite surface thereof. When in phase however, that is initially at and close to the source of the waves, the two Lamb waves combine to produce a quasi Rayleigh wave, as can be seen from a comparison of FIGS. 1E and 1F to FIG. 1D. As the two Lamb wave modes travel further from the source, due to the differing phase velocities and the resultant phase difference between them, there is a complete transference of wave energy from the outer surface on which the transducer, generating the wave, is mounted to the opposite outer surface. This transference of energy between the outer surfaces of the plate occurs at regularly spaced intervals, making a touch plate having large enough dimensions for this transference to occur, unsuitable for a touch position sensor.
From the above, it is seen that touch position sensors as shown in the above mentioned patents utilizing surface acoustic waves and more particularly quasi-Rayleigh waves, as is necessary for these sensors to operate, are limited to relatively thick panels, i.e., panels having a thickness of three to four times the wavelength of the surface acoustic wave propagating therein. Further, quasi-Rayleigh waves are confined at and near a single surface. The consequence of using quasi-Rayleigh waves according to the above patents leads to several undesirable attributes, namely excessive sensitivity to contaminants or other materials abutting the touch panel, and excessive panel weight and thickness for many applications. The excessive sensitivity to contamination is due to the confinement of wave energy at or near the confining surface. As a result the quasi-Rayleigh wave energy or a large fraction thereof is absorbed by even modest amounts of surface contaminants. The effect of near or total absorption of wave energy by contamination, sealants or other materials abutting the plate, is to create acoustic shadows or blind spots extending along the axes that intersect the contaminant. A touch position sensor according to the above mentioned patents cannot detect touch if one or both coordinates is on a blinded axis. In a sense, touch panels utilizing quasi-Rayleigh waves are unduly sensitive to contamination or abutting materials. The scope for optimizing the performance of a touch sensor according to the above patents is limited because touch sensitivity and minimum touch panel thickness are not independent choices. In order to support a quasi-Rayleigh wave in a touch panel of reduced thickness, its other dimensions remaining the same, the wavelength must be reduced to preserve single surface confinement. It is characteristic of Rayleigh/quasi Rayleigh waves that their confinement depth is related to wavelength, with confinement depth decreasing as the wavelength is reduced. As a result, the wave is confined to a shallower region bounded by the surface, and the proportion of wave energy absorbed by a given absorbing medium is increased. Experimentally this is found to vary, approximately by the inverse square of the wavelength. As discussed previously, touch sensors according to above patents can be considered unduly sensitive for some applications, even for relatively thick panels, hence the effect of reducing touch panel thickness results in touch sensors even more sensitive to surface contamination and other abutments. Conversely, reducing sensitivity by increasing the quasi-Rayleigh wavelength results in increased panel thickness and weight. Losses in wave energy over distance as a result of air damping of the surface acoustic wave is also significant since surface acoustic waves are confined to the surface of the touch plate. The energy losses due to air damping further limit the size of the touch plate.
As shown in FIGS. 1A and C, surface acoustic waves are imparted into a touch plate utilizing a transducer mounted on a wedge that is in turn mounted on the touch surface of the plate wherein the transducer vibrates in the direction shown to produce a compressional bulk wave that propagates in the wedge to impart a surface acoustic wave in the touch plate. This type of wave generating device has several drawbacks. Because the device must convert a compressional bulk wave to a surface acoustic wave, the efficiency of the device is not as high in practice as if the transducer produced waves were of the same type as those imparted into the plate, i.e., via direct conversion. Also, because the wedge extends above the plate, it must be accommodated for in mounting the plate. Wedges are typically made of plastic thus creating a difficulty in bonding the wedge to a glass plate. Further, the transducer must be bonded to the wedge and the wedge then bonded to the touch plate. Because problems with reliability increase with the number of bonds required, this surface acoustic wave generating device is not as reliable as other wave generating devices requiring fewer bonds.
Although acoustic waves other than surface acoustic wave can propagate in a solid, such waves including Lamb waves and shear waves, heretofore these other acoustic waves were thought to be unsuitable for a touch position sensor. Lamb waves were thought unsuitable because they are dispersive, varying in phase and velocity, so as to interfere with one another. Shear waves were thought unsuitable because a touch on a plate in which shear waves are propagating absorbs only a small percentage of the shear wave energy intercepted by a touched surface whereas a touch on a plate in which a surface acoustic wave is propagating absorbs a much greater percentage of the intercepted surface acoustic wave energy. More particularly, the percentage of intercepted energy absorbed by a given touch can be over ten times greater for a surface acoustic wave than it is for a zeroth order horizontally polarized shear wave for practical touch plate thickness. Since shear waves are not nearly as responsive to touch as surface acoustic waves, shear waves were not thought practical for a touch position sensor.